1. Field of the Invention
This invention relates generally to tape recorders and playback machines and, more particularly, is directed to determining the longitudinal time code addresses of selected information signals recorded on tapes.
2. Description of the Prior Art
There are many functions of tape recorders, particularly video tape recorders (VTRs), for which it is desirable to easily and accurately determine the positions or addresses of selected signals recorded upon a tape. For example, such addresses can be used to subsequently relocate desired signals, and, in the field of broadcast and commercial video tape recording, in which sophisticated machinery has been developed for the control of VTRs, such addresses are used to automatically cause VTRs to cue to, and to initiate and terminate playback at specified tape positions. In video tape editing, such addresses may be used to control the relatively complicated task of automatically causing playback and record VTRs to start up, achieve synchronization with each other, and simultaneously reach respective preselected tape addresses at which the signal from the playback VTR commences to be recorded on the tape in the record VTR. Such addresses may also be used in video editing to determine when the edit ends, to preview the edit, to initiate slow motion or the operation of special effect generators and to cause various of the signals which can be recorded on a VTR, such as the video and one or more audio signals, to be cut in or out at different times.
In order to satisfy the desire of VTR users, particularly broadcasters and commercial video tape recorders, for means to accurately select the addresses of selected locations on video tape recordings, video tape machines have been produced which allow an operator to rapidly locate and view the recorded signal corresponding to a desired video frame. For example, many VTRs and VTR editors provide a shuttle mode of operation in which an operator can view the visual information recorded on the tape while moving the tape at a controllably variable speed, which ranges between speeds that are substantially higher and substantially lower than normal speed. In addition, many such machines also have a jog mode of operation in which the operator can view the visual information recorded on the tape while the tape is moved either very slowly or not at all.
Although controls which allow slow or still motion playback, such as shuttle and jog controls, make it easy to find, view and select a desired frame of a video recording, they do not solve the problem of making it possible to easily and accurately assign a tape address to the selected frame. To do that, apparatus must be provided for easily and accurately determining the current tape address when the desired frame is viewed.
One method which has been used in the past to determine such a current tape address is the counting of control pulses. Control pulses or control track signals are recorded serially along many video tapes for the purpose of synchronizing the motion of such tapes relative to the moving or rotary transducer heads which are used to record and reproduce video signals thereon. Since each recording format records a predetermined number of such control pulses in conjunction with each frame, counting of the control pulses can provide an accurate relative address for each frame. Because of their relatively low frequency, commonly either one or two control pulses are recorded per frame, such control pulses are relatively easy to read and, although they cannot be read when a tape is virtually or actually at a stand-still, they can be read and counted at modest tape speeds, and particularly at all but the lowest tape speeds encountered in the shuttle or jog modes. Thus, tape control equipment which counts control pulses can provide a fairly accurate relative address of a frame viewed at slow or still motion.
Unfortunately, determining tape addresses by counting control pulses has several major disadvantages. Most important is the fact that the counting of control pulses provides nothing more than a measure of the distance along the tape occupied by the counted control pulses and thus can only provide a tape address that is relative to a point on the tape from which the control pulses have been continuously counted. In order to obtain an absolute tape address, it is necessary to set the counter at zero or another predetermined value at a known location, usually near the start of the tape, and to continuously count control pulses from such known location. If, for any reason, the count value is lost, it is necessary to return to the known position and reset the counter. As the tape is moved over greater distances, and as it is stopped and started and alternately moved in forward and reverse directions, as tapes often are in the editing process, it becomes increasingly likely that an occasional control pulse will be miscounted, owing to drop out, random noise, or the movement of control pulses past the control pulse reading head at a time when the motion of the tape is too low for the control pulse to be reliably read. Each miscount adversely affects the accuracy of all subsequently counted tape addresses until the count is again reset.
Because of the foregoing problems associated with determining tape addresses by counting control pulses, professional or broadcast video tape equipment often provides means for recording and reading time code addresses on a video tape, which addresses separately identify each frame of a recorded video signal. Such time codes commonly identify each frame of the video signal in terms of a sequential frame number, which specifies an hour, a minute, a second, and a frame value. For example, in the Society of Motion Picture and Television Engineers (SMPTE) time code for National Television System Commission (NTSC) video signals, frame numbers vary sequentially from a lowest possible value of 00 hours, 00 minutes, 00 seconds, and 00 frames, to a maximum possible value of 23 hours, 59 minutes, 59 seconds, and 29 frames.
There are two major types of time codes, namely, longitudinal time codes which are recorded in a track along a longitudinal margin of the tape, and vertical interval time codes which are recorded in the same tracks as the video signal, but in the portions of such tracks corresponding to the vertical blanking periods or intervals.
When longitudinal time codes can be read, they provide a very reliable and accurate means for identifying the address of each individual frame. Unfortunately, longitudinal time codes often cannot be read reliably, if at all, during slow or still motion playback because the output of conventional playback or reproducing heads is proportional to the rate of change of the flux acting on the head from the respective track, and not to the flux itself. Thus, with prior art longitudinal time code equipment, an operator is usually unable to directly determine the longitudinal time code address of a desired video frame selected while moving a tape at a slow speed, for example, while using a jog or shuttle mode. As a result, in order to find the longitudinal time code address of a desired video frame, it is often necessary to repeatedly perform the operation of adding or subtracting frames from an approximate time code address and viewing the frame having the resulting address until such viewing indicates that the correct time code address has been selected.
As is well known in the art of video editing, when vertical interval time codes are used to address video tapes, the equipment required for producing and reading such vertical interval time codes is more expensive than that required for longitudinal time codes. Many VTR's, particularly those equipped with so-called dynamic tracking, encounter difficulties in accurately reading such vertical interval time codes in slow or still motion modes.